char* toLowerCase(char* s) {
    int num = strlen(s);
    char k = 'a' - 'A';
    for (int i = 0; i < num; i++)
    {
        if (s[i] >= 'A' && s[i] <= 'Z')
        {
            s[i] += k;
        }
    }

    return s;
}